This invention relates to biaxially oriented rigid conduit or extrudate characterized by having a unique deformed spherulitic microstructure and improved physical and mechanical properties and to biaxially oriented flexible film which retains the unique microstructure and improved physical properties of the extrudate and to a method for producing the conduit and film.
As discussed in the prior applications, relatively thick, rigid thermoplastic polymer sheet can be produced by solid state hydrostatic extrusion of a thermoplastic polymer preform into a rigid, thick-walled conduit having a desired wall thickness, slitting the conduit and heat-flattening the slit conduit. It is also possible to produce such sheet by solid state hydrostatic extrusion of a slit thermoplastic polymer preform to produce a slit extrudate and heat-flattening the extrudate. However, it has not been readily possible to produce thermoplastic polymer flexible films by this described method.
In the method for producing thick, rigid thermoplastic polymer sheet by solid state hydrostatic extrusion a thermoplastic crystalline polymer preform is heated to a desired temperature between its 4.64 kilograms of force per square centimeter deflection temperature and 8.degree. C. below its crystalline melt temperature and is extruded by hydrostatic pressure through an extrusion zone having converging walls, a converging cross-section and a diverging geometry. The extrusion pressure in the fluid is maintained by sealing means which allows a thin layer of fluid to be extruded on the outer surfaces of the preform to act as a lubricant. The preform is expanded circumferentially and elongated axially to produce a rigid conduit or extrudate having a relatively thick wall. The extrudate is cooled as it emerges from the expansion zone. Cooling stabilizes the polymer and reduces its inherent tendency to spring back to its original shape.
During extrusion the wall thickness of the preform is decreased and the extrusion pressure increases because the ratio of the volume of the preform divided by the contact area in the extrusion zone decreases, thereby causing the extrusion zone-to-workpiece friction to consume a greater portion of the process energy. Hence, increased pressure for extrusion is required. Also, because the preform is heat softened the extrudate exiting from the extrusion zone is hot and relatively soft and becomes flexible due to the reduced wall thickness and is very difficult to handle without wrinkling or distortion. This difficulty could be overcome by stretching the extrudate but such stretching would destroy the spherulitic crystalline aggregate structure of the thermoplastic crystalline polymer. Therefore, stretching cannot be applied to an extrudate if the spherulitic crystalline aggregate structure of the polymer is to be retained.
In order to maintain satisfactory uniformity of extrudate thickness it is necessary to maintain the dimensional tolerances for the tooling which comprises the extrusion zone. Such dimensional tolerances become even more critical when extruding relatively thin walls and the setup and alignment of the tooling becomes critical and expensive.
If the extrudate exit velocity remains constant, the throughput rate in pounds per hour drops proportionately to the extrudate thickness hence the production costs per unit weight are increased.
In short, the complexity of the equipment required to produce polymer film by solid state hydrostatic extrusion becomes greater and the cost of production prohibitive. Additionally, production costs increase as the thickness of the polymer extrudate decreases.
In accordance with this invention the above problems can be avoided and polymer film can be produced by solid state hydrostatic extrusion by using a preform which is comprised of a plurality of concentric tubular-like discrete continguous layers of thermoplastic crystalline polymers and forming a multilayer, rigid extrudate whose layers are concentric flexible films.
Thermoplastic polymer film conventionally is produced by the blown tubular or flat die methods. In the blown tubular or bubble method, molten polymer is extruded vertically generally in an upward direction through an annular orifice or ring die into tubular form which is solidified and passed through nip rolls which apply tension to the polymer and flatten the tube. The tube is collected on windup rolls. The molten polymer is cooled as it leaves the annular orifice by a fluid, usually air, blown against its outer surface and by a gaseous bubble formed in the interior of the tubular extrudate. The circumferential expansion of the polymer by the bubble and the axially elongation by the nip rolls stretch the polymer to produce a biaxially oriented film. The film width and thickness are controlled by regulating the opening of the annular orifice. The unconstrained circumferential and axial stretching of the polymer results in at least partial destruction of the original spherulitic crystalline aggregate structure of the polymer.
In the production of polymer film by the flat die method, molten polymer is extruded through a flat or "T"-shaped die into water or over several chill rolls to solidify and "set" the polymer. The solidified film is passed over another series of rolls and through nip rolls to apply tension to the film. The speed of collecting the film on take-up rolls results in film thinning and results in uniaxial orientation of the polymer structure. Film so produced can be biaxially oriented by stretching the film in a direction substantially perpendicular to the axial direction. Tensioning or stretching the film does orient the structure but also destroys the original spherulitic crystalline aggregate structure of the polymer.
Very thin film, for example as thin as 0.0001 inch, can be produced by the conventional processes described above but such film is difficult to handle. Special techniques and equipment are required to maintain film flatness and to prevent excessive rejects during processing, thereby increasing the costs of producing such film.
It is known that by orienting the structure of a thermoplastic crystalline polymer both the physical and mechanical properties of the polymer are improved. Orientation, whether uniaxial or biaxial, has always been achieved by tensioning or stretching methods as noted above. Stretching or tensioning processes cause non-homogeneous deformation of the original spherulitic crystalline aggregate structure of the thermoplastic crystalline polymer. As stress is applied to the polymer, initially the aggregates are deformed elastically. As stress is increased the aggregates are elongated, tilted and eventually disrupted and separation of lamellae occurs. The appearance of the original spherulitic crystalline aggregate structure is thus more or less destroyed. Microvoids, microfibrils and eventually fibrils are formed in the polymer. Defects, such as microvoids, which may be present in the extruded molten polymer, are aggravated. If the polymer contains filler material, for example mineral particles and glass fibers, stretching can cause the matrix polymer to be separated from the filler material causing microvoids and discontinuities in the sheet.
There is no known method for producing thermoplastic crystalline polymer film which retains the original spherical crystalline aggregate structure of the polymer while orienting the polymer structure and to improve its properties. Nor is there a known method to produce a filled thermoplastic crystalline polymer film in which the resin matrix microstructure is oriented.
It is therefore the object of this invention to provide thermoplastic polymer film having a substantially uniform thickness within a range between about 0.005 and 0.03 of an inch (0.13 mm to 0.76 mm) and which is characterized by a structure comprised of spherulitic crystalline aggregates which are biaxially oriented in the plane of the film and are compressed in a plane transverse to the plane of the film, has improved ultimate tensile strength, improved tensile impact strength particularly at low temperatures, improved thermal conductivity in the plane of the film and permeability through the film which is less than the non-oriented polymer or a film of comparable biaxially oriented structure produced by stretch processing and a density at least equal to the density of the non-oriented polymer.
It is a further object of this invention to provide a method for producing such biaxially oriented films by solid state hydrostatic extrusion.
It is also another object of this invention to provide a method for producing such thermoplastic crystalline polymer film having increased orientation and improved properties wherein a multilayered polymer preform is initially extruded by solid state hydrostatic extrusion into a tubular multilayered rigid extrudate and re-extruding the extrudate at least once again to form a multilayered extrudate whose layers are separable into flexible films.
It is another object of this invention to provide a thermoplastic crystalline polymer film which contains at least 5 weight percent and as much as 80 weight percent particulate filler material, which film is characterized by having a biaxially oriented spherulitic crystalline aggregate structure and which is substantially free from process-induced defects.
It is still another object of this invention to provide a rigid, thick-walled, multi-layered polymer conduit the layers of which are concentric, tubular, discrete, flexible films, each film being biaxially oriented and which film may contain between 5 and 80 weight percent particulate filler material, each film being characterized by a structure comprised of spherulitic crystalline aggregates biaxially oriented in the plane of the film and compressed in a plane transverse to the plane of the film and having improved ultimate tensile strength and tensile impact properties particularly at low temperature and improved thermal conductivity in the plane of the film.